High speed, automated processing machines now employed in mills for paper manufacture require a high degree of consistency control. This control is necessary because variations in feedstock consistency may result in dramatic changes in the finished product. If not strictly monitored, such variations will destroy the uniformity and, therefore, the desirability of the finished paper product.
In order to monitor feedstock consistency, thereby minimizng such variations, a number of devices have been adopted in the paper industry. Devices dedicated to this purpose may be categorized as four primary types: non-invasive, in-line; invasive, in-line; non-invasive, off-line; and invasive, off-line.
Turning first to the non-invasive, invasive distinction, the non-invasive devices are generally more modern and contemplate the use of ultrasound or light, which generally detect consistency variations by comparative analysis with known standards. Such devices are exemplified by that illustrated in U.S. Pat. No. 4,171,916. In-line, non-invasive measuring devices dedicated to other purposes are also known. For example, Heine, in U.S. Pat. No. 4,285,239, describes a device for determining the density of flowing slurry materials.
Returning to consistency measuring devices, others employ non-invasive pressure transducers to make comparative analysis like that depicted in Staege, U.S. Pat. No. 2,627,788. The most common invasive type of device is characterized by an impeller. Impeller devices often are based on comparative driving shaft torque measurements to indicate fluctuating consistency of stock (See Coats, U.S. Pat. No. 3,155,866). Another type of impeller-based measuring device is illustrated in Madsen, U.S. Pat. No. 4,148,214, having pressure transducers located in close proximity to the impeller blades to detect pressure differences and, consequently, consistency variations. Impeller-based devices are also employed in off-line devices. Cowan, in U.S. Pat. No. 3,528,281, employs an impeller to draw fluid from a conduit into a sample tube where the variable volume flow is used to determine consistency. Staege constitutes an off-line device which employs non-invasive apparatus for paper stock pressure measurement.
All of the above-described consistency measuring devices determine consistency by empirical comparative analysis. The impeller-based devices are recognized to give repeatable measurements and, if associated with a control device, are generally capable of regulating consistency to .+-.0.05%. However, these devices suffer from two noted shortcomings. First, although repeatable, the measurements are often inaccurate. Secondly, impeller-type devices often become snagged with string, other strong fibrous materials or fabric pieces. Hence, frequent cleaning and recalibration are the rule. The non-invasive wave energy frequency type (ultrasound, light, etc.) often produce less repeatable measurements (.+-.0.1%) due to fluctuations in fiber length and flow rate. Excepting Coats, all recognize a relationship between consistency and pressure and/or velocity but determine the relationship in comparative empirical analysis.
In view of the noted shortcomings of currently available consistency measuring and control devices and the considerable efforts to perfect such devices, the need still exists for accurate, repeatable consistency measurement.